1. Field of the Invention
The present invention relates to a hydroplane that is propelled by the jet of water ejected rearward by the rotation of an impeller.
2. Description of the Prior Art
A typical small-type hydroplane has a two-cycle engine and a jet pump (both not illustrated) mounted in its hull and can move in the predetermined direction by driving the two-cycle engine so as to power the jet pump. The reason for the mounting of a two-cycle engine in the hull is based on the fact that it needs neither an oil pan mechanism nor valve gear mechanism and it is of light weight and compact and can produce a high specific power, all of these features making it suitable for small-type hydroplanes.
However, a two-cycle engine has a large fuel consumption and exhausts a large amount of hydrocarbons (to be abbreviated as HCs hereinbelow) so that it is not a good way to respond to requests for prevention of air, river, lake and sea pollution. In recent years, four-cycle engines, which have a reduced fuel consumption and exhaust a lower amount of HCs, have been developed and improved by raising the revolution speed, increasing the engine displacement and using a multicylinder configuration in order to ensure output power comparable to that of two-cycle engines as well as to prevent environmental pollution. In particular, the effect of raising the revolution speed of four-cycle engines, has made the realization of compact, light-weight, high power four-cycle engines possible.
Prior art concerning hydroplanes of this type are found in Japanese Patent Application Laid-Open Hei 7 No.237,586, Japanese Patent Application Laid-Open Hei 7 No.237,587, Japanese Patent Application Laid-Open Hei 7 No.237,588, Japanese Patent Application Laid-Open Hei 8 No.26,185, Japanese Patent Application Laid-Open Hei 8 No.49,596 and Japanese Patent Application Laid-Open Hei 8 No.53,098.
As stated above, the conventional hydroplanes use four-cycle engines which have been developed and improved by raising the revolution speed and increasing the engine displacement and by using a multicylinder configuration in order to ensure output power comparable to that of two-cycle engines. In this case, the exhaust system heavily affects the output power, and hence needs a very complicated shape and configuration in order to meet the required performances. Accordingly, it has been very difficult to produce a light-weight, compact exhaust system for it.